1. Field of the Invention
The invention relates to a hydraulic circuit for longwall mining by means of a support device (support shield) for underground mining according to the various embodiments described herein.
2. Description of Related Art
Such circuits are generally known and in use. They are hydraulic systems controlled by inherent system pressure. The pump pressure in the hydraulic circulating system is also used for the pilot control of the valves. This approach has become dominant in control technology for mining. It enables the use of only two supply lines to the longwall. In contrast to this approach, systems with outside control generate the hydraulic pilot control commands via separate control valves which work independently of the load pressure or pump pressure, wherein the control valves are supplied with pressurizing agent via separate pressure lines. Such a system necessarily involves a separate line for the returning control materials. This also increases the amount and complexity of tubing in the system. When problems arise, it is very difficult to locate the problem, because it cannot be ruled out that both pressure supply lines—meaning the working pressure supply line and the pilot control pressure line—are influencing each other mutually. Also, the valves have a significantly more complex technical construction, particularly with respect to pressure equalizing functions and seals, due to the requirement that the working pressure not be coupled to the pilot control pressure.
On the other hand, the high standards for safety in the mining industry, which also apply to hydraulic systems controlled by inherent system pressure, require significant inputs for safety measures. These inputs are made more complex by the high complexity and the number of control elements and switching elements in a longwall operation, particularly control valves and load holding valves. The primary requirement is that hydraulic systems designed for shielded mining methods must ensure the safety of underground personnel below the supports in the longwall area, despite the large number of possible operating conditions, including unacceptable operating conditions or unplanned disruptions.
WO2005054629 discloses a comparable hydraulic circuit, wherein the pressure sensors monitor the presence of a minimal pressure in the longwall supply line 1, said line connecting the hydraulically operated cylinder/piston units, as well as the hydraulic control valves assigned thereto, to the pump via a pump branch line. Said pressure sensors are connected to the electrical control unit (5) to allow shutting off of the system.
This design ensures that the hydraulic control valves assigned to the cylinder/piston units are only operated once the inherent system pressure has built to a sufficient degree to support the roof of the mine, particularly upon startup of the pumps. However, after normal startup of the pumps, it is not possible to prevent the controllable check valve—which holds the cylinder/piston unit against the roof pressure—from receiving a wrongful control command, particularly not in the case of irregularities within a single support shield.
DE 10 2004 017 712 A1 discloses a comparable hydraulic circuit, wherein the power source in each support shield, along with its associated control valve, is connected to the return line via a shared common return line, and a group blocking valve which is assigned to the support shield is switched closed depending on the hydraulic current in the common return line when the hydraulic current falls below a threshold value. In this case, a volume current is the basis for switching closed the valve at the support shield without pressure. However, as such, it is not possible to prevent irregular pressure states in the supply or return, such that it is possible that undesired and wrongful switching can occur, particularly unblocking of the check valve which holds the roof pressure.
Therefore, despite these safety measures, it has been observed that undesired functions can be triggered or executed, and negatively influence personnel safety.
Relevant causes may include: loose valve seals, which release the pressure applied on the holding cylinders or working cylinders of the support shield(s); open valves or valves that are jammed open; jammed-open pilot valves; blockage in the return line, which can result in the undesired opening of safety valves or load holding valves/releasable check valves (depending on the area ratio of the control piston), and/or which can extend the cylinder/piston units.
The problem addressed by the invention is that of designing the hydraulic systems which are currently in operation and are controlled by their own system pressure in such a manner that the aforementioned life-threatening and costly disruptions cannot occur, and also in such a manner that it is possible to retrofit existing systems without significant conversion inputs.